Problem 27 - Chemistry Matter and Change

Question

Determine Which of the following processes are exothermic? Endothermic? a. \(C_{2} H_{5} O H(1) \rightarrow C_{2} H_{5} O H(g)\) d. \({NH}_{3}({g}) \rightarrow {NH}_{3}({l})\) b. \(B r_{2}(1) \rightarrow B r_{2}(s)\) e. \({NaCl}({s}) \rightarrow {NaCl}({l})\) c. \({C}_{5} {H}_{12}({g})+8 {O}_{2}({g}) \rightarrow 5 {CO}_{2}({g})+6 {H}_{2} {O}({l})\)

Step-by-Step Solution

Answer

a. Endothermic b. Exothermic c. Exothermic d. Exothermic e. Endothermic

Step 1: Process a

Since the process for a is: \[C_{2}H_{5}OH(l) \rightarrow C_{2}H_{5}OH(g)\] it is undergoing a change from liquid to gas, which requires the absorption of heat or energy, so this process is endothermic.

Step 2: Process b

The process for b is: \[Br_{2}(l) \rightarrow Br_{2}(s)\] As the substance is changing from liquid to solid, it is releasing energy in the form of heat, making this process exothermic.

Step 3: Process c

The process for c is a combustion reaction: \[C_5H_{12}(g) + 8O_2(g) \rightarrow 5CO_2(g) + 6H_2O(l)\] Combustion reactions are typically exothermic, because they release energy in the form of heat and/or light.

Step 4: Process d

The process for d is: \[NH_3(g) \rightarrow NH_3(l)\] As ammonia changes from a gas to a liquid, energy is being released in the process. Therefore, this is an exothermic process.

Step 5: Process e

The process for e is: \[NaCl(s) \rightarrow NaCl(l)\] In this case, the substance is undergoing a phase change from solid to liquid, which requires the absorption of heat or energy. Thus, this process is endothermic.

Key Concepts

Phase Change

When a substance changes its phase, that is from a solid to a liquid, liquid to gas, or vice versa, it either absorbs or releases energy. This energy change is a pivotal concept in thermodynamics and is closely tied to the particle motion within the substance.

For instance, when a liquid becomes a gas, as in the case alcohol evaporating \(C_{2}H_{5}OH(l) \rightarrow C_{2}H_{5}OH(g)\), the process requires energy to break the intermolecular forces that hold the liquid molecules together. The heat absorbed during this transition makes it an endothermic process. In contrast, when bromine cools from a liquid to a solid \(Br_{2}(l) \rightarrow Br_{2}(s)\), it releases energy to the surroundings since the molecules settle into a more stable state, making this an exothermic process.

Combustion Reactions

Combustion reactions are a class of chemical reactions where a substance, typically a fuel, reacts with an oxidant, commonly oxygen, releasing energy in the form of heat and light. The general formula for a combustion reaction is fuel + oxygen \rightarrow combustion products + energy.

An example of this is the combustion of pentane \(C_5H_{12}(g) + 8O_2(g) \rightarrow 5CO_2(g) + 6H_2O(l)\). This process is highly exothermic as it releases a significant amount of heat, making it a cornerstone for engines and industrial processes where energy generation is required.

Energy Absorption

The concept of energy absorption is central to endothermic processes. During these processes, energy is taken in from the surroundings into the system, typically resulting in a temperature decrease in the external environment.

This energy is often required to break bonds or overcome intermolecular forces, as we see in the melting of sodium chloride \(NaCl(s) \rightarrow NaCl(l)\). Here, energy is needed to disrupt the lattice structure of the solid ionic compound, resulting in the formation of the liquid phase. Such processes are critical to understand when designing systems for cooling or for reactions that require energy input to proceed.

Heat Release

Exothermic processes are characterized by the release of heat as a product transitions to a lower energy state. This release can occur when chemical bonds are formed or when a substance undergoes a phase transition to a more ordered state.

For example, the freezing of liquid bromine into solid bromine is exothermic. Likewise, the condensation of ammonia gas \(NH_3(g) \rightarrow NH_3(l)\) liberates heat as the gas particles come closer together to form the denser liquid. This heat release can be harnessed in various technologies, from harnessing geothermal energy to creating self-heating containers and hand warmers.